1. Field of the Invention
The present invention relates to a shape measuring system and method wherein an intensity-modulated light is emitted toward an object and the distance up to the object is measured on the basis of a phase difference between the light reflected from the object and the emitted light. Particularly, the present invention is concerned with a shape measuring system of reduced size and cost and a shape measuring method both capable of measuring the distance up to an object accurately and independently of external conditions such as a change in reflectance of the object surface.
2. Description of the Prior Art
As methods for measuring a three-dimensional shape there have been proposed two methods which are a passive method and an active method. The passive method measures the shape of an object without radiating energy to the object, while the active method measures the shape of an object by radiating a certain energy to the object and detecting its reflection.
As an example of the passive method there is known a stereo method which measures distances at plural points up to an object. According to the stereo method, two cameras are disposed at a certain interval and the distance up to an object is determined by triangulation on the basis of a parallax of two images obtained. This method is advantageous in that a remote distance can be measured if only images can be picked up, but involves a serious problem that it is impossible to make a three-dimensional measurement for the whole of a smooth surface free of any pattern. In addition, since it is basically impossible to align the optical axes of two cameras with each other, there has been a drawback that there occurs an area (occlusion) incapable of measuring distance.
As to the active method, a light-stripe method is mentioned as an example of the method which measures distances at plural points up to an object. According to this light-stripe method, a slit light is radiated to an object at a certain angle and the distance up to the object is determined by triangulation on the basis of an image picked up at an angle different from the said radiation angle. This method is characteristic in that a relatively simple construction suffices, but it is required that the slit light be scanned in a very small angular unit, resulting in that the measurement time is long because an image is picked up at every angular unit. For solving this problem there has been proposed a structured light method which utilizes the light-stripe method. According to the structured light method, a pattern of a projection light is coded instead of radiating the slit light many times and it is thereby intended to measure a distance with a reduced number of projections. However, given that the number of samples in the horizontal direction is n, it is necessary that image pick-up be conducted log2 n times (nine times if n=512 points), thus giving rise to the problem that the measurement time becomes longer. Additionally, since it is basically impossible to align the optical axis of a projector and that of an image pick-up device with each other, a drawback has been encountered that there occurs an area (occlusion) incapable of measuring a distance.
In connection with the active method, as an example of the method capable of measuring distances at plural points by one image pick-up, there is known a phase distribution measuring method wherein an intensity-modulated light is radiated to an object and a phase distribution of reflected light is measured.
As examples of conventional phase distribution measuring methods are mentioned methods disclosed in Literature 1, xe2x80x9cAn article described on pages 126 to 134 of SPIE Vol. 2588 (1995) (A new active 3D-Vision system based on rf-modulation interferometry light),xe2x80x9d Japanese Patent No. 2690673, and SPIE Vol. 2748 (1996), pp. 47-59, xe2x80x9cThe Emerging Versatility of a Scannless Range Imager.xe2x80x9d
FIG. 10 illustrates the conventional shape measuring system described in Literature 1. This shape measuring system, indicated at 100, comprises a modulation/demodulation signal generator 104 which applies an intensity modulation to light emitted from a light source 101A through a condenser lens 102 to a plane modulator 103 using such a crystal as Pockels cell, a projection lens 106 which projects an intensity-modulated light 105a onto an object 6 planewise, the modulation/demodulation signal generator 104 which applies an intensity demodulation to a reflected light 105b incident on a plane demodulator 108 through a focusing lens 107 after being reflected by the object 6, the plane demodulator 108 using such a crystal as Pockels cell, and a CCD camera 109 which picks up an intensity-demodulated light signal. According to this construction, light emitted from the light source 101A is directed to the plane modulator 103 by the condenser lens 102 and is intensity-modulated in accordance with a signal produced from the modulation/demodulation signal generator 104. Thereafter, the intensity-modulated signal 105a is projected planewise onto the object 6 by the projection lens 106. The reflected light 105b from the object 6 is introduced into the plane demodulator 108 by the focusing lens 107 and is intensity demodulated in accordance with a signal provided from the modulation/demodulation signal generator 104, then is focused on the CCD camera 109. An intensity image picked up by the CCD camera 109 contains a phase information based on the distance up to the object 6. By processing this intensity image with a computer 110 it is possible to obtain distance data of the object 6 at a single pick-up of image.
FIG. 11 illustrates the conventional shape measuring system disclosed in Japanese Patent No. 2690673. This shape measuring system is different in the following three points from the shape measuring system shown in FIG. 10. The first point is the use of a semiconductor laser 101B as a light source, the second point is that intensity modulation is performed directly by the semiconductor laser 101B without the use of a modulator using such a crystal as Pockels cell, and the third point is that demodulation is performed by an image intensifier 111 without the use of a demodulator using such a crystal as Pockels cell. Light which has been subjected to intensity modulation in accordance with a signal provided from the modulation/demodulation signal generator 104 is radiated from the semiconductor laser 101B and is then projected planewise onto the object 6 by the projection lens 106. Reflected light 105b from the object 6 is focused on the image intensifier 111 by the focusing lens 107. The signal from the modulation/demodulation signal generator 104 is converted to a high-voltage signal by a high-voltage drive circuit 112, which signal is inputted to a gain controller terminal of the image intensifier 111. Thus, the intensity-demodulated reflected light is picked up by the CCD camera 109. A intensity image obtained in the CCD camera 109 contains phase information based on the distance up to the object 6. By processing this intensity image with the computer 110 it is possible to obtain distance data of the object 6 at a single pick-up of image.
However, the shape measuring system shown in FIG. 10 is disadvantageous in that the cost thereof is very high because a modulator/demodulator using such a crystal as Pockels cell is used for each of the plane modulator 103 and the plane demodulator 108. Moreover, since the modulator/demodulator using such a crystal is of a small aperture which is several millimeters or so, it is necessary that the light emitted from the light source 101A and the light reflected by the object 6 be condensed in conformity with the said aperture by the condenser lens 107, with consequent increase in system size.
The shape measuring system shown in FIG. 11 is also disadvantageous in that it is very expensive because the image intensifier 111 is used. Moreover, for driving the image intensifier 111 it is necessary to intensity-modulate a signal whose voltage is as high as several hundred volts, thus giving rise to a disadvantage that, the structure of a drive circuit used is complicated. Further, the whole of the system becomes larger in size because the image intensifier 111 is larger than the CCD camera 109.
As a distance measuring method using neither the expensive modulator/demodulator nor the expensive and large-sized image intensifier, a phase distribution measuring method using a reference light is disclosed, for example, in Japanese Published Examined Patent Application No. Sho 59-30233.
FIG. 12 illustrates a conventional shape measuring system which adopts the method just mentioned above. This shape measuring system, indicated at 100, comprises a light emitting element 123 adapted to emit light after intensity modulation at a predetermined frequency with a drive signal which is provided from a drive circuit 121 and which is based on a signal produced from an oscillator 120, a beam splitter 125 adapted to transmit and reflect light which is incident thereon from the light emitting element 123 through a projection lens 124, a light receiving element 127 adapted to receive a light reflected again by the beam splitter 125 after passage through the beam splitter 125 and after subsequent reflection by the object 6 and a light which has passed through the beam splitter 125 again after being reflected by the beam splitter 125 and after subsequent reflection by a reflecting mirror 128, an amplifier 128 which amplifies an output signal provided from the light receiving element 127, a wave detector 129 which detects an output signal provided from the amplifier 128, and a level meter 130 which reads an amplitude from an output signal provided from the wave detector 129.
In such a configuration, when from the light emitting element 123 is emitted a light after intensity modulation at a predetermined frequency with a drive signal which is provided from the drive circuit 121 and which is based on a signal produced from the oscillator 120, the light is directed to the beam splitter 125 through the projection lens 124. One light (illumination light) which has passed through the beam splitter 125 is reflected by an object 6 and then reflected by the splitter 125 and is introduced into the light receiving element 127 through the condenser lens 126. The other light (reference light) reflected by the beam splitter 125 is then reflected by the reflecting mirror 128 which is disposed at a known distance, then passes through the beam splitter 125 and enters the light receiving element 127. The illumination light and reference light are optically combined on the light receiving element 127 and the thus-combined light is converted its waveform into an electric signal, which is applied to the amplifier 128. The amplitude of this waveform varies depending on the difference between the distance from the light receiving element 127 to the object 6 and the distance from the light receiving element 127 to the reflecting mirror 128. The thus-amplified waveform signal is detected by the wave detector 129 and the amplitude thereof is read by the level meter 130, whereby the distance up to the object 6 can be calculated.
According to the above conventional shape measuring system, however, since the illumination light and the reference light each pass through the beam splitter 125 twice in the section from the light emitting element 123 to the light receiving element 127, the quantity of light incident on the light receiving element 127 decreases to one fourth, thus giving rise to the problem that the light utilization efficiency is poor.
Moreover, since the light incident on the light receiving element 127 is influenced by reflectance on the surface of the object 6 and the distance up to the object 6 is calculated on the basis of an output signal provided from the light receiving element 127, there arises the problem that an accurate distance cannot be measured due to the difference in reflectance of the object surface.
The conventional shape measuring system in question further involves the following serious problems. For an object having concave and convex with respect to an optical axis it is impossible to make an accurate measurement of distance. Accurate distance cannot be measured where extraneous light is present although no problem arises in the dark. For the measurement of a three-dimensional shape it is necessary that light be scanned in two dimensions, resulting in increase of the measurement time. Thus, the measurement of distance is infeasible in ordinary environmental conditions.
Accordingly, present invention provides a shape measuring system which is small in size, low in cost and high in the light utilization efficiency.
The present invention also provides a shape measuring system and method capable of measuring the distance up to an object accurately without being influenced by external conditions such as a change in reflectance of an object surface.
The present invention also provides a shape measuring system wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency toward the object, a reflecting member that reflects in a predetermined direction a part of the light emitted from the light emitting unit, and a detecting unit that receives the light reflected from the object and also receives the light reflected by said reflecting member and that outputs a detection signal with the said phase difference reflected therein.
In the above construction, as the reflecting member there may be used, for example, a beam splitter which transmits and reflects the emitted light at a predetermined ratio or a reflecting mirror disposed at a position not obstructing the radiation of emitted light to the object. The use of a beam splitter as the reflecting member is advantageous in that the emitted light passes through the beam splitter only once and that therefore the quantity of the reflected light and that of the emitted light both received by the detecting unit are prevented from decrease by the beam splitter. Likewise, the use of a reflecting mirror as the reflecting member is advantageous in that the quantity of the reflected light and that of the emitted light both received by the detecting member are prevented from decrease by the reflecting mirror.
The present invention also provides a shape measuring system wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency or a stationary light not intensity-modulated toward the object, a reflecting member that reflects in a predetermined direction a part of the light emitted from the light emitting unit or a part of the stationary light, a detecting unit that receives the light reflected from the object and also receives the light reflected by the reflecting member and that outputs a composite detection signal produced by combining both received lights and with the said phase difference reflected therein, the detecting unit further receiving the stationary light reflected by the object and outputting a reflected stationary light detection signal, the detecting unit further receiving the stationary light from the reflecting member and outputting a stationary light detection signal, and a calculating unit that, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal, makes correction for eliminating external components such as a change in reflectance of the object and calculates the said distance.
According to this construction, the distance up to the object can be determined accurately by eliminating external components such as a change in reflectance of the object.
The present invention further provides a shape measuring system wherein light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring system comprising a light emitting unit that emits the light having been intensity-modulated at the predetermined frequency or a stationary light not intensity-modulated toward the object, a detecting unit that receives the light reflected from the object and also receives the emitted light and outputs a composite detection signal produced by combining both received lights and with the said phase difference reflected therein, and then receives the emitted stationary light after the reflection by the object and outputs a reflected stationary light detection signal, and further receives the stationary light and outputs a stationary light detection signal, and a calculating unit that, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal, makes correction for eliminating external components such as a change in reflectance of the object and calculates the said distance.
According to this construction, even in such an optical system as shown in FIG. 12, the distance up to the object can be determined accurately by removing an external component such as a change in reflectance of the object.
The present invention further provides a shape measuring method wherein a light having been intensity-modulated at a predetermined frequency is emitted toward an object and the distance up to the object is determined on the basis of a phase difference between reflected light from the object and the emitted light, the shape measuring method comprising a first step and a second step, the first step comprising emitting the light intensity-modulated at the predetermined frequency toward the object, detecting the reflected light and the emitted light, combining both lights into a composite detection signal with the phase difference reflected therein, emitting a stationary light not intensity-modulated toward the object, detecting the stationary light reflected from the object, converting the reflected stationary light thus detected into a reflected stationary light detection signal, and detecting the stationary light and converting it into a stationary light detection signal, and the second step comprising making correction for eliminating external components such as a change in reflectance of the object and calculating the said distance, in accordance with the said composite detection signal, the said reflected stationary light detection signal and the said stationary light detection signal.
According to this construction, the removal of external components such as a change in reflectance of the object permits the distance up to the object to be determined accurately.